[metapost] Using |s| as a variable name

[nicola]

Hello,
I am a bit puzzled by the way a name like |s| may behave. Why is this 
seemingly correct:

   string s;
   s = "aaaaaaa";
   |s| = length s;
   show |s|;

and why does the following produce an error?

   string s;
   s = "aaaaaaa";
   |s| = length s;
   for i = 1 upto |s|:
      ;
   endfor;

>> |s|:
! Improper final value has been replaced by 0.

I would like to use |s| in a subroutine, as follows:

vardef foo(expr x) =
   |x| := length x;
enddef;

string s;
s = "abc";
foo(s);
end;

Why is this not possible? Isn't |x| a valid sequence of tags?

Nicola